1. Field of the Invention
The present invention relates to hair dyeing by iridoid glycosides and aglycons thereof. More particularly, the present invention provides a method of dyeing hair by a naturally occuring iridoid glycoside, an aglycon thereof or a semisynthetic analog of said glycoside, and a composition for use in such method.
2. Background Information
Various dyes have been used for dyeing white hair or for changing color of hair. Among such dyes, the most widely used are the oxidative hair dyes, the active component of which has, however, sensitizing and toxic properties and thus necessitate care in actual application. Organic and inorganic pigments have also been used. These are, however, easily removed on washing hair and repeated dyeing would be necessary. Another examples used for dyeing hair are acid dyes which require dyeing assistant such as benzyl alcohol in order to improve dyeing efficacy. Anyway, the conventional dyes are not satisfactory in safety requirements for application onto human hair.
Further, when the conventional dyes are used for dyeing hair, a considerably long time such as more than one hour is required for dyeing (including washing). In addition, finish of dyeing at the border of the hair is uneven and white hairs appear unevenly. Simple applying of pigment, so-called fashionable hair dyeing, is certainly easy but the applied pigment can be easily come loose and fall by touching with hand. In contrast, according to the present invention, long-lasting dyeing can be attained by simple dyeing procedure such as once or twice a day applying of the invented composition and a good finish of dyeing is obtained at the border of the hair.
While it has been known that the iridoid glycosides react with primary amines to form color substances and that such substances can be used for coloring foods, medicaments or cosmetics (for example, JP-A-53934/1977, JP-A-155259/1982, JP-A-125573/1988 and 19234/1986), it has not been known that the iridoid glycosides themselves can be used for dyeing hair.